Radio frequency photocathode electron guns are the source of choice for most high-performance accelerator systems. The main reason for this popularity is their ability to produce very bright beams of electrons. However, due to inherent limitations, photocathode radio frequency electron guns have not successfully penetrated certain key applications. One of these limitations is their inability to economically produce the high average current, high brightness electron beams necessary for certain applications. Another drawback is that one must choose between high quantum efficiency and durability. Durable cathodes tend to have relatively low quantum-efficiency, while high quantum efficiency cathode materials are very sensitive to vacuum conditions.
Superconducting Radio Frequency injectors are highly sought after for high brightness, high duty factor electron sources. The major hurdle in its development is the lack of a suitable photocathode that has high quantum efficiency, long life time and is compatible with the superconductivity of the injector.
Although generation of electrons from metals using multiphoton photoemission by use of nanostructured plasmonic surfaces has been reported for copper and aluminum, these structures are not suitable for forming fully superconducting radio frequency injectors. Furthermore, the aluminium nanostructures are grooves which unfortunately are sensitive to the polarization of the laser.
Accordingly, it would be desirable to provide a photocathode that has high quantum efficiency, long life time, and is compatible with a superconducting radio frequency injector.